The present invention relates to a vehicle motion control system for controlling the motion of a vehicle in a stable manner even when it is steered successively in opposite directions.
Conventionally known is a vehicle motion control system which determines a dynamic variable of the vehicle as the vehicle turns a corner and controls the motion of the vehicle according to the deviation of the dynamic variable from a target value of the dynamic variable. For instance, according to a known yaw rate control system, a deviation of a yaw rate actually measured by using a yaw rate sensor from a target yaw rate computed from the steering angle is converted into a yaw moment that would eliminate the deviation, and the right and left wheels are braked individually and differently so as to produce a corresponding yaw moment. For instance, when the vehicle has an oversteer tendency, the outer wheels are braked more than the inner wheels. Conversely, when the vehicle has an understeer tendency, the inner wheels are braked more than the outer wheels.
However, according to such a conventional vehicle motion control system, when the vehicle is brought back from a state involving a large slip angle to a stable state either by the intervention of the vehicle operator or automatically by the system, an overshoot in the yaw movement of the vehicle tends to be produced due to the inertia of the vehicle, and even an oscillatory yaw movement may result.
In view of such problems of the prior art, a primary object of the present invention is to provide a vehicle motion control system based on a target yaw rate scheme which ensures a stable motion to the vehicle under all conditions.
A second object of the present invention is to provide a vehicle motion control system based on a target yaw rate scheme which ensures a stable motion to the vehicle without requiring any significant change to the existing system.
According to the present invention, these and other objects of the present invention can be accomplished by providing a vehicle motion control system for controlling a motion of a vehicle during cornering, comprising: a steering angle sensor for detecting a steering angle; a yaw rate sensor for detecting an actual vehicle yaw rate; an actual yaw rate increment computing unit for computing an increment of the actual yaw rate; a vehicle body slip angle computing unit for estimating a vehicle body slip angle; a brake force computing unit for controlling brake forces of right and left front wheels of the vehicle; and a counter steer detecting unit for detecting a counter steer action according to the steering angle and yaw rate; the brake force computing unit being adapted to brake an outer front wheel in a controlled manner when a sign of the yaw rate has changed and the yaw rate increment has become equal to or greater than a threshold value after a counter steer action has been detected and the vehicle body slip angle has become equal to or greater than a threshold value.
Thus, suppose that a vehicle travels a winding road and a counter steer action is taken. If the vehicle body slip angle and actual yaw rate increment exceed threshold values, the outer front wheel is braked. Therefore, even when the vehicle body slip angle reaches a maximum value and an attempt is made to reduce it again by a counter steer action (in a similar manner as a swinging pendulum), because the increases in the yaw rate and actual yaw rate increment are predicted and monitored before the vehicle body slip angle reaches its maximum value, the vehicle is enabled to travel in a stable fashion.
Preferably, the counter steer detecting unit is adapted to detect a counter steer action when signs of the steering angle and yaw rate differ from each other.